1. Field of the Invention
This invention relates to optical imaging systems and specifically concerns an optical system for transforming a collimated light beam into a self-luminous source of arbitrary shape and numerical aperture.
2. Description of the Prior Art
The drive toward higher density circuitry in microelectronic devices has promoted interest in a variety of high resolution lithographic techniques which require the ability to produce finer resolution patterns at high production rates. In optical lithography, the improvement in resolution that results by use of shorter wavelengths is well known. As a result of these considerations, an effort has been made to develop processes and materials which require exposure in the deep UV spectral region. The light source traditionally used in these prior art systems has been either a deuterium lamp or a xenon-mercury arc lamp. The problem with using such lamps is that insufficient power is available from them in the desired spectral region. For a typical lamp in a typical system, the total deep UV power that can be collected for use is in the few tens of milliwatts range, so that the exposure time for resists that are sensitive in the deep UV are typically several minutes.
In the prior art systems, an aperture stop is provided so that, by varying the size of the aperture, the cone angle of illumination is varied and, thus, the degree of partial coherence and accompanying edge resolution are also varied. However, since the aperture stop also limits the total amount of light entering the projection system, longer exposure times are required to achieve the enhanced edge resolution.